"A terrible tragedy happened at Bioce and everything is being done to reduce the number of casualties as much as possible," President Vujanovic told reporters.

"The whole republic has been mobilised to save lives. We are all hoping that this terrible accident will have as few casualties as possible," he added.

Serbia and Montenegro's rail network has suffered from years of under-investment but is used by much of the population simply because it is the cheapest method of travel, the BBC's Balkans correspondent Matt Prodger says.

Montenegro is the smaller of two republics making up Serbia and Montenegro, the only two republics of the former Yugoslavia still joined together.